This project addresses the need for alternative marking methods to replace the potential loss of oxytetracycline as an approved marking compound for fish. This study is significant because otolith marking has proven to be an effective tool to determine the hatchery origin of individual fish in high seas, coastal waters, and the lakes and rivers around the globe. This study will test whether fish immersed in strontium chloride hexahydrate for three times the standard duration (24 hours) and one, three and five times the standard concentration (3000 ppm) will show adverse signs, mortalities, gross or histological lesions. The target animal safety study of strontium chloride hexahydrate will be conducted with Pacific salmon fry. Based on a literature review, strontium chloride hexahydrate appears to be an ideal chemical mark. This project will provide needed safety data to support approval for a new marking compound, strontium chloride hexahydrate. The current application addresses the need to develop alternative, reliable, and cost-effective methods of marking otoliths in hatchery-raised salmonids as well as other cultured fish species. Otoliths are used by fish for balance, orientation and sound detection. They function similarly to the inner ear of mammals. Chemical marking involves incorporating biologically rare elements or compounds into body tissues and is an efficient means of marking large numbers of small fish. An otolith's ring structure can provide fisheries biologists with information about an individual's age, growth rate, and environment. Otoliths also supply information on life history, recruitment, and taxonomy, which are why they are widely used in fisheries management. Marking fish allows managers to determine if stocked fish are contributing to recruitment. The advantages of strontium chloride hexahydrate include 100% effectiveness, relatively short exposure times, generation of long lasting marks any time after hatch, and reasonable costs.